X-linked agammaglobulinemia (XLA) is a human genetic disorder due to a mutation in the gene for Bruton's tyrosine kinase (Btk) that results in the near absence of peripheral B lymphocytes. The CBA/N mouse also has an X-linked immunodeficiency (XID) from a point mutation in murine Btk, which reduces peripheral B cell number, serum immunoglobulin, and B cell signal transduction efficiency. How mutations in BTK result in immune dysfunction is not completely understood. The investigator has recently demonstrated that the reduced peripheral B cell pool of xid mice results from the dysregulation of programmed cell death (PCD). PCD in mature resting lymphocytes is normally suppressed by Bcl-2. He found that in xid B cells the concentration of Bcl-2 protein and the extent of its phosphorylation is significantly reduced relative to normal B cells. Overexpression of a Bcl-2 transgene expanded the follicular B cell population normally deficient in xid mice but signal transduction defects of xid B cells remained. These data were used to develop a hypothesis that the peripheral long-lived B cell pool is maintained by an active process in which Btk-mediated signals regulate Bcl-2 expression, and/or its posttranslational modification to suppress PCD. Further, that dysregulation of peripheral lymphocyte survival can be a basis for primary immunodeficiencies. In AIM 1, the investigator will determine how ligands in the in vivo microenvironment can regulate B cell survival by Btk dependent signaling. In AIM 2, the investigator will attempt to correct xid by gene replacement therapy using a retroviral vector and peripheral xid B cells as gene recipients. This provides an opportunity to directly determine how the survival and antigen responsiveness of mature B cells is dependent on Btk, if mutant Btk protein will exert a dominant negative effect on the newly introduced wild type Btk, if other members of the Btk family can substitute for Btk, and if wt Btk-mediated signaling will correctly modulate Bcl-2 expression/function in transduced xid B cells. These studies will develop an animal model system where critical approaches and pitfalls of gene therapy can be directly tested, and determine the suitability of long-lived mature lymphocytes as vehicles for gene therapy.